This invention relates to the production of ethylene copolymers under a pressure of at least 200 Kg/cm.sup.2 and at a temperature of at least 125.degree. C.
Recently, as disclosed in British Patent No. 828,828, processes for polymerizing ethylene at high temperature and pressure by ionic coordination polymerization using a Ziegler-type catalyst, with the use of a polymerization apparatus for high-pressure polyethylene have been proposed.
Also, it is currently known to copolymerize ethylene with an .alpha.-olefin by this process, whereby the density of the ethylene copolymer is controlled and a linear low-density polyethylene (LLDPE) is produced.
In the industrial production of the LLDPE, the above mentioned process is very advantageous in that an existing apparatus for production of high-pressure polyethylene can be utilized as it is, and new investment in the apparatus is not needed.
The following problems, however, have existed when an ethylene copolymer is produced at high temperature and pressure with the use of a Ziegler-type catalyst, in comparison with the conventional process using this catalyst at low temperature and pressure.
Firstly, the ethylene copolymer produced at high temperature and pressure contains a large amount of a fraction extractable with boiling hexane. Such a copolymer causes blocking and the like when processed into films, etc., and also causes food-sanitation problems. The formation of the extractable fraction mainly depends on a low-crystalline copolymer wherein an .alpha.-olefin is copolymerized ununiformly.
Secondly, such a copolymer contains more double bonds in the polymer than the copolymer produced at low temperature and pressure. When a polymer contains a large amount of double bonds therein and especially in its polymer chain, the polymer is susceptible to oxidative decomposition and exhibits poor weathering properties.
Thus, development of a novel catalyst which solves the above described problems in the production of an ethylene copolymer to be polymerized with a Ziegler-type catalyst at high temperature and pressure has been desired.